1. Field of the Invention
This invention relates to a bicycle speed control apparatus. More particularly, this invention relates to a bicycle speed control apparatus provided with a pair of shifting levers to control a gear transmission mechanism via a drive cable.
2. Description of Prior Art
U.S. Pat. No. 5,287,766 is disclosed with a speed control apparatus that is provided with a shift lever attached to the handlebar of a bicycle so as to control a derailleur (not shown). It is quite simple to produce the shift lever and assemble it with other elements. Because the shifter is positioned by frictional force, the shifter is easily disengaged and the chain cannot be appropriately positioned when the bicycle passes over an uneven road. Thus, the rider often feels discomfort on his/her palm because of the excessive shifting range generated by the single shift lever during the shifting process.
In U.S. Pat. No. 5,438,889, the driving force acted on the lever portion of the shifter has to be generated by the external pulley assembly (cable winding reel (16), drive reel portion (16) and the drive cable (K)), and therefore the driving force cannot be precisely controlled and the shifter cannot be properly positioned at a predetermined site when it is pushed outside the range.
In Taiwan Patent No. 276229 discloses two complicate mechanisms, each connected to the shifter and used to determine the movement of the shifter. Because the assembled mechanisms can be provided with larger driving force, the cable connected between the shifter and the two complicate mechanisms gradually extends over time. Then, the tract of the shift gradually becomes insufficient and the shifting process may be interrupted unexpectedly.
To solve the above problem, the primary object of this invention is to provide a bicycle speed control apparatus so as to shift precisely a gearbox or a gear transmission mechanism of a bicycle through a drive cable. The bicycle speed control apparatus of the present invention comprises a first seat connected to a bicycle handlebar, and a second seat provided with a shaft connected to the first seat so as to construct the main structure thereof. A cylindrical sleeve member is movably disposed about the shaft and connected to the end of a drive cable. The cylindrical sleeve member has an inner wall provided with first teeth, second teeth and third teeth, the third teeth located between the first teeth and the second teeth in parallel. A first shifting lever and a second shifting lever for respectively shifting a front and rear speed shifter of the gear transmission mechanism, are provided to pivot on the shaft, each of which having a hole used for receiving the shaft. A first pawl and a second pawl are provided to movably mount on the first shifting lever and the second shifting lever, respectively. The first pawl is formed with a first thrust end so as to engage the first teeth, and the second pawl is formed with a second thrust end so as to engage the second teeth. A resilient member positioned on the shaft so as to engage the third teeth and disengage the first pawl from the first teeth and disengaging the second pawl from the second teeth. The resilient member has a first protrusion for pressing the first pawl to disengage from the first teeth and a second protrusion for pressing the second pawl to disengage from the second teeth and a third protrusion for engaging the third teeth. A regulator is connected to and adjusts the tension of the drive cable.